摘要:

A previously developed analytic model for charge carrier recombination in bilayer organic light emitting diodes [D. V. Khramtchenkov, V. I. Arkhipov, and H. Ba¨ssler, J. Appl. Phys.79, 9283 (1996)] in which charge transport across the interface between anodic and cathodic cell compartments is impeded by energy barriers is extended to cells of arbitrary thickness of the constituent layers and variable energy barriers. The results indicate that the recombination yield is the result of a complicated interplay between redistribution of the electric field affecting the injection at the electrodes, internal charge accumulation, and field assisted barrier crossing. Unit charge carrier recombination efficiency is predicted to occur at moderate cell voltages and electron injection barriers less than≈0.4 eV.At higher voltages, leakage process across the interface becomes progressively important. With increasing electron-injection barrier, that injection process becomes rate limiting. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365258

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Diodes consisting of a hole transporting layer containing tri-stilbeneamine as active medium and a hole blocking layer (an oxadiazole blend) sandwiched between indium-tinoxide and aluminium electrodes have been characterized via both theirj(F)characteristics and the dependence of intensity, quantum efficiency, and spectrum of the electroluminescence on the thicknessLeof the hole blocking layer. The results are discussed within the framework of the analytic model advanced in the preceding paper. Both the decrease of the effect of electric field screening at the anode due to the space charge accumulation at the internal interface as well as the recombination efficiency with decreasingLeis in good agreement with theory. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365259

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We report the observation of oscillatory features (OFs) in the photoreflectance (PR) and contactless electroreflectance (CER) spectra of CdTe films grown on Si substrate, at energies below the band gap of CdTe. The simultaneous observation of OF in the reflectance(R)spectrum having the same period as those in the PR and CER spectra (as also their dependence on film thickness) provides a direct proof for optical interference effects as being the source of these features. However, in the present case the amplitude of the OF gets damped towards shorter wavelengths while remaining nearly wavelength independent in the longer wavelength region indicating a modulation mechanism different from those reported earlier. A series of experiments and simulations performed by us seem to indicate that while in PR the principal mechanism is the pump beam induced periodic temperature changes which in turn modulates the optical path length of the CdTe film, in CER the mechanism is the electric field induced modulation of the subband gap refractive index of the film through the electro-optic effect. The damping of the OF has been explained on the basis of subband gap absorption by the CdTe film. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365260

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Thin solid films of closely packed CdS nanocrystals with sizes between 1.5 and 1.8 nm have been prepared and sandwiched between indium–tin–oxide (ITO) and silver contacts. At room temperature, these CdS nanocrystals start to emit light visible to the unaided eye at a bias above 15 V. The spectral characteristics are voltage controlled and tunable between 1.8 eV and 2.8 eV with increasing bias from 20 to 40 V. The current–voltage dependence indicates hopping conductivity of the carriers injected. At high voltage the electroluminescence and photoluminescence spectra are similar showing a strong red shift of the luminescence maximum with respect to the absorption peak at 3.75 eV which we explain by an effective radiative recombination through deep traps in the interface region. Consequently, the tunability of electroluminescence is attributed to excitation of different deep traps by electron transport across neighbouring CdS nanocrystals. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365261

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have undertaken a systematic study of porous silicon with the use of photoconductivity and photoluminescence. During this, we have observed at least three kinds of time-varying photoconductivity, of which two reduce the conductivity and one increases the conductivity during illumination. In addition, we have observed persistent photoconductivity. The time developments of the photoconductivity as well as the persistent photoconductivity depend in intricate ways on parameters such as the wavelength and intensity of the illuminating light, the potential drop across the sample, the surface treatment, and the dark current value. The time scale of these time-varying effects ranges from a few minutes to several hours. The results are discussed in terms of the photoelectric properties of the supporting silicon wafer, diffusion of hydrogen, and photoinduced desorption of hydrogen. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365262

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The degradation behavior of the sulfur-treated InP surface at relatively low temperature has been investigated with x-ray photoelectron and photoluminescence (PL) spectroscopy. The results showed that the treated surfaces were oxidized toIn2O3,InPO3, andInPO4at 250 °C and in a vacuum of10−3Torr for 20 min. As the holding time for S-treated InP under a vacuum of10−3Torr increased, the PL peak caused by the band edge transition decreased without the formation of oxides. It was therefore suggested that the decrease of the PL intensity for S-treated InP is only related to the generation of phosphorous vacancies at the surface, not to oxide formation. The usefulness of a thin S overlayer on III–V semiconductors was also discussed. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365263

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Medium energy ion scattering has been used to study the role of nitrogen in the thermal oxidation kinetics of ultrathin silicon oxynitrides. Oxynitride films with different amounts of nitrogen near theSiOxNy/Siinterface and pure (control)SiO2/Sifilms were reoxidized in dry18O2under equivalent conditions. The spatial distribution of18Oincorporated into the films was analyzed by high-resolution depth profiling methods. Analogous to the pureSiO2case, we observed two distinct regions where oxygen incorporation into the oxynitride films occurs: at/near the interface and near the outer oxide surface. The (near) interface oxide growth reaction is found to be significantly retarded by the presence of near-interfacial nitrogen (with a higher degree of the retardation for higher concentrations of nitrogen). The presence of nitrogen near the interface does not affect the surface exchange reaction. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365264

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The growth of anodic oxides of crystalline Si in anO2plasma has been studied. The kinetics are found to follow those expected when neutral O atoms in the plasma capture an electron at the substrate surface and diffuse through the growing oxide under the influence of an applied positive electric field. The oxides have been characterized using infrared spectroscopy, glancing incidence x-ray reflectometry, and electric capacitance/voltage measurements. Shifts to lower wave numbers of the transverse and longitudinal optic infrared modes associated with the asymmetric stretch of the bridging O’s with respect to thermally grown oxide values are observed. We conclude that these shifts result from structural modifications induced by ultraviolet photons present in the O plasma. A simple model suggests that the plasma grown oxides have a density∼3.8&percent;larger than thermalSiO2and an average Si–O–Si bridging bond angle∼1.6°smaller. The plasma grown oxides are inhomogeneous in the volume, the bond angle shift increasing to∼−2.3°,and the density decreasing to+2&percent;with respect to thermal oxide near the Si substrate/SiO2interface. X-ray reflectivity data, which are consistent with film inhomogeneity, enable us to determine the film thickness and surface (air/SiO2) roughness. The latter is always ⩽1 nm. Electrical measurements suggest that the oxides contain fixed oxide charges and interface states which can be reduced by low temperature annealing. The leakage currents remain high, however. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365235

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

Si oxidation promoted by a platinum (Pt) overlayer has been investigated using x-ray photoelectron spectroscopy and synchrotron radiation ultraviolet photoelectron spectroscopy. Heat treatments of the specimens with ⟨∼5-nm-Pt/0.5–1-nm-chemical oxide/Si(100)⟩ structure at 300–400 °C increase the oxide thickness to 4–5 nm. The amounts of the suboxide species,a(Si+),a(Si2+),anda(Si3+),in the chemical oxide layers formed in hydrochloric acid (HCl) plus hydrogen peroxide(H2O2)are in the order ofa(Si+)>a(Si2+)>a(Si3+),while those for the oxide layers formed in nitric acid(HNO3)have an order ofa(Si3+)>a(Si2+)≈a(Si+).The amounts of the suboxide species in the former oxide layers are much higher than those in the latter oxide layers. These results indicate that theHNO3oxide layers are more highly oxidized, probably resulting in a higher atomic density and a lower defect density. Although the initial chemical oxide layers formed inHCl+H2O2are thinner than those grown inHNO3,the former oxide layers become thicker than the latter after the Pt deposition and the heat treatments below 200 °C. This result is attributed to the lower atomic density and the higher defect density of the chemical oxide layers produced inHCl+H2O2,which enhance the diffusion of oxidizing species. It is suggested that the variation in the atomic density of the chemical oxide layers cause the different energy distribution of interface states in the Si band gap. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365265

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

We have investigated the influence of the (001) GaAs substrate preparation on the first stages of ZnSe heteroepitaxial growth by molecular beam epitaxy. We show that three different GaAs reconstructions occur depending on theex situsubstrate preparation, the Se residual pressure in the growth chamber and the temperature of heating. After deoxidation, an epiready substrate leads to a(2×1)-reconstructed surface at high temperature(∼600 °C)which turns into an unreconstructed surface when cooling down to the growth temperature (280 °C). An etched substrate, on the other hand, exhibits a(2×3)or a(4×3)reconstruction depending on the temperature reached during deoxidation. Both reconstructions are stable upon cooling down to the growth temperature. Direct nucleation of ZnSe on such deoxidized substrates leads to three-dimensional (3D), quasi two-dimensional (2D) and purely 2D growth modes on the unreconstructed,(2×3)and(4×3)reconstructed surfaces, respectively. Very pronounced oscillations of the reflection high-energy electron diffraction intensity are observed during nucleation on the(4×3)surface. Excellent agreement is obtained between simulated and experimental x-ray rocking curves for pseudomorphic layers grown on a(4×3)starting surface. In addition, their low-temperature photoluminescence spectra are dominated by free exciton recombinations without any defect-related line. Our results thus demonstrate that we have achieved a substantial improvement of ZnSe heteroepitaxy on bare GaAs substrates. ©1997 American Institute of Physics.

ISSN:0021-8979    

DOI:10.1063/1.365266

出版商:AIP    

年代:1997

数据来源: AIP